Mechanism of Ultrasonic Cavitation Nucleation in Liquid Helium by Quantized Vortices

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Abstract
The interaction between an ultrasonic field and one or a few quantized vortex lines in He II was investigated using the audible cavitation threshold as a means of detection. Experiments involved both rotating ultrasonic buckets and two shafts, each with a paddle attached, rotatable in the same or opposite sense near an ultrasonic field. Histograms of cavitation threshold measurements were not Gaussian, but appeared to be bimodal. The high threshold mode was most frequent in the quiescent state, but the lower mode was most frequent with rotation. The threshold values were found to fluctuate with time both with and without rotation. Slight and gradual reductions of audible threshold were observed in the rotating bucket sometimes, beginning at speeds less than that predicted by the Arkhipov-Vinen formula. Reductions at speeds higher than the Arkhipov-Vinen value were not directly proportional to the total number of vortices believed to be present in the bucket. Two shafts with paddles rotating in opposite senses caused larger reductions in threshold than a single shaft and paddle; but this was not the case when the shafts rotated in the same sense.